The amounts of trace elements, such as potassium, copper, or zinc, that can be detected in human hair depends not only upon their concentration in the body but also upon how quickly the hair has been growing. Thus, when illness, deficiency, or dieting temporarily slows hair growth, little potassium and almost no copper or zinc gets into the hair, even though blood levels of these elements remain within normal limits.
Hair analysis, according to correspondence in the Lancet (2:608), is really only useful in detecting certain substances, such as lead or arsenic, when they have been in the body in excessive amounts for some time. Hair analysis is not really capable of demonstrating transient chemical changes in the tissues, nor is it a useful tool in assessing general health or nutritional status. Many people, unfortunately, undergo hair analysis believing that it provides valuable information that cannot be obtained easily by other means.